Typically an automatic tensioner consists of a spring and a damper system. The spring is used to maintain a quasi-constant tension in the system, while the damper is used to reduce dynamic vibrations and to keep the pulley continuously in contact with the belt. If undampened, the tensioner would vibrate excessively under the dynamic influence of the engine.
Existing hydraulic tensioners can be divided into two groups: strut-type hydraulic tensioners and multi-disc viscous tensioners. Strut-type tensioners are less suitable because they have a very high unidirectional damping and the unit acts as a ratchet, pumping up the tensioner at resonance and causing increased dynamic belt tension. Also, these strut-type tensioners require a relatively large packaging space which is usually not available in the timing drive area of an engine. Multi-disc tensioners generate approximately the same damping level in both directions and thus are not able to continuously follow the driving element. In addition, both types of tensioners require a complicated installation procedure and are difficult to service after installation in the field.
U.S. Pat. No. 4,721,495 discloses an auto tensioner having an interior chamber filled with hydraulic fluid. The tensioner comprises a fixed portion and a displaceable portion pivotable relative to the fixed portion. Each of the fixed portion and the displaceable portion have a plurality of vanes extending into the fluid chamber. Relatively large clearances are provided between the ends of the vanes and the adjacent surface finding the fluid chamber. When increased or decreased tension in the belt causes the displaceable portion to move relative to the fixed portion, the vanes move relative to one another and cause the hydraulic fluid to flow through these clearances in a restricted manor, thereby damping movement of the displaceable portion.
One problem with the tensioner disclosed in the '495 patent is that the damping provided by the fluid is substantially the same in either direction of movement. Specifically, it will be noted that the vanes are provided symmetrically such that the fluid flow between the clearances thereof will be restricted in the same manner regardless of whether the displaceable portion is moving under an increased or decreased belt tension. In many situations it is desirable to have damping characteristics which are different in one direction than they are in the other. In particular, it is desired that the damping be greater in the belt tensioned direction than in the decreased belt tension direction. This arrangement helps prevent belts slippage which can be caused by belt becoming slack during decreased belt tension and the tensioner arm not moving quickly enough to apply the appropriate tensioning.
It is therefore an object of the present invention to provide a belt tensioner which provides different damping characteristics in opposed directions. In order to achieve this objective, the present invention provides a belt tensioner for use in an engine. The tensioner comprises a fixed structure constructed and arranged to be fixed to the engine. A movable structure is mounted for movement relative to the fixed structure in a belt tensioning direction and an opposite direction. A pulley member is rotatably mounted on the movable structure. The pulley member has a belt engaging surface positioned and configured to be engaged with the endless belt such that movement of the belt rotates the pulley member. One of the fixed structure and the movable structure has an interior surface defining a fluid chamber containing substantially incompressible fluid. The other of the fixed structure and the movable structure includes a chamber dividing structure disposed within the fluid chamber. The chamber dividing structure cooperates with the interior surface defining the fluid chamber so as to define first and second chamber portions within the fluid chamber on opposing sides of the chamber dividing structure.
A biasing element is engaged with the movable structure. The biasing element applies a biasing force to bias the movable structure in the tensioning direction to tension the belt. The chamber dividing structure is constructed and arranged such that the relative movement of the movable structure in the tensioning direction displaces fluid from the first chamber portion to the second chamber portion and increases fluid pressure in the first chamber portion and decreases fluid pressure in the second chamber portion, and relative movement of the movable structure in the opposite direction displaces fluid from the second chamber portion to the first chamber portion and increases fluid pressure in the second chamber portion and decreases fluid pressure in the first chamber portion. The chamber dividing structure being configured to allow the fluid to flow between the chamber portions in a restricted manner so as to yieldingly resist the relative movement of the movable structure and thereby dampen the relative movement of the movable structure. The chamber dividing structure is constructed and arranged such that the fluid flow restriction is greater when the movable structure moves in the opposite direction than when the movable structure moves in the tensioning direction, thereby providing the movable structure with greater resistance to movement in the opposite direction than in the tensioning direction.
Preferably, the aforesaid one of the fixed and moveable structures is the moveable structure and the other of the fixed and moveable structures is the fixed structure. It can be appreciated that the tensioner constructed in accordance with this aspect of the present invention provides different dampen characteristics in opposing directions. The characteristics can be selected for particular applications.
In order to provide the restricted flow, a plurality of passageways may be formed through the dividing structure and a plurality of flapper plates be positioned so as to close the passageways. A number of the flapper plates are disposed on one side of the dividing structure and the remainder are disposed on the other side of the dividing structure.
Another problem with the tensioner of the '495 patent is that it has no way of compensating for increased fluid pressure due to increased operating temperatures. With the tensioner mounted to the engine during operation, the engine gives off a substantial amount of heat, which in turn elevates the fluid temperature and accordingly increases fluid pressure. This increased pressure can have an adverse effect on tensioner performance.
Therefore, it is another object of the present invention to provide a belt tensioner which is adapted to compensate for increased fluid temperature and the resultant fluid pressure increase. In order to achieve this object, another aspect of the present invention provides a belt tensioner for use in an engine. The tensioner comprises a fixed structure constructed and arranged to be fixed to the engine. A movable structure is mounted for movement relative to the fixed structure in a tensioning direction and an opposite direction. A pulley member is rotatably mounted on the movable structure. The pulley member has a belt engaging surface positioned and configured to be engaged with the belt such that movement of the belt rotates the pulley member. One of the fixed structure and the movable structure has an interior surface defining a fluid chamber containing substantially incompressible fluid. The other of the fixed structure and the movable structure includes a chamber dividing structure disposed within the fluid chamber. The chamber dividing structure cooperates with the interior surface defining the fluid chamber so as to define a first and second chamber portions within the fluid chamber on opposing sides of the chamber dividing structure.
A biasing element is engaged with the movable structure. The biasing element applies a biasing force to bias the movable structure in the tensioning direction so as to tension the belt. The chamber dividing structure is constructed and arranged such that the relative movement of the movable structure in the tensioning direction displaces fluid from the first chamber portion to the second chamber portion and increases fluid pressure in the first chamber portion and decreases fluid pressure in the second chamber portion, and relative movement of the movable structure in the opposite direction displaces fluid from the second chamber portion to the first chamber portion and increases fluid pressure in the second chamber portion and decreases fluid pressure in the first chamber portion. The chamber dividing structure is configured to allow the fluid to flow between the chamber portions in a restricted manner so as to yieldingly resist the relative movement of the movable structure and thereby dampen the relative movement of the movable structure.
A surface defines a volume compensation chamber communicated to the fluid chamber. A resiliently compressible structure is disposed inside the compensation chamber. The compressible structure and the compensation chamber are positioned and configured such that the substantially incompressible fluid can flow from the fluid chamber to the compensation chamber when the fluid pressure in the chamber increases as a result of increased temperature such that the fluid compresses the resiliently compressible structure so as to volumetrically expand the compensation chamber and compensate for the increased fluid pressure.
Other objects, features, and advantages of the present invention will be initiated from the following detailed description, the accompanying drawings, and the appended claims.